The present invention relates to technical field of a cup seal to be used in a master cylinder of a brake or a clutch of a vehicle such as an automobile, or the like and to a technical field of a plunger-type master cylinder employing the cup seal.
Conventionally, a master cylinder for developing fluid pressure according to pedaling force on a brake pedal or a clutch pedal is used in a hydraulic brake system or a hydraulic clutch system for the purpose of actuating a brake or a clutch. As such a master cylinder, a plunger-type master cylinder is known from Japanese Unexamined Patent Publication No. 2003-194100, which comprises a cylinder body having a cylinder bore, a piston which is slidably inserted into the cylinder bore and defines a fluid pressure chamber, a communication path which is formed in the cylinder body and communicates with a reservoir, a relief port which is formed in the piston to allow communication between the communication path and the fluid pressure chamber, and a sealing member which is received in a concavity formed in the inner periphery of the cylinder bore of the cylinder body and into which the piston is slidably inserted so as to seal between the inner periphery of the cylinder bore and the outer periphery of the piston.
When the plunger-type master cylinder is inoperative, the communication between the relief port of the piston and the communication path is not isolated by the sealing member so that the fluid pressure chamber communicates with the reservoir via the relief port and the communication path. Therefore, in the inoperative state, the fluid pressure chamber is at atmospheric pressure so that no fluid pressure is developed. As the piston moves forward toward the fluid pressure chamber side by depression of a clutch pedal, the communication between the relief port and the communication path is isolated by the sealing member so that the fluid pressure chamber is isolated from the reservoir. In this manner, according to the forward movement of the piston, fluid pressure is developed in the fluid pressure chamber.
Generally, such a sealing member used in the plunger-type master cylinder is required to have a sealing function for preventing leakage of fluid pressure during development of fluid pressure by the forward movement of the piston and also required to have a pumping function as a fluid supplying function for supplying brake fluid of the reservoir to the fluid pressure chamber for the purpose of improving responsiveness of the piston during the retraction. To impart the both functions to a sealing member, a cup seal is employed as the sealing member. The cup seal is formed to have a laterally-facing U-shaped section and comprises an annular base portion extending radially, an inner lip axially extending from the inner peripheral side end of the base portion, and an outer lip axially extending from the outer peripheral side end of the base portion.
When fluid pressure is developed in the fluid pressure chamber, the inner lip is pressed into close contact with the outer periphery of the piston by the fluid pressure and the outer lip is pressed into close contact with a bottom wall of the concavity, in which the sealing member is received, by the fluid pressure, whereby the sealing member fluid-tightly seals between the outer periphery of the piston and the inner periphery of the cylinder bore.
Since the volume of the fluid pressure chamber is increased when the piston is retracted after development of the fluid pressure, the pressure in the fluid pressure chamber is reduced and tends to be negative pressure. Accordingly, the inner lip is deflected outwards apart form the outer periphery of the piston so as to form a space therebetween and the outer lip is deflected inwards apart from the bottom wall of the concavity so as to form a space therebetween. Through these spaces, brake fluid of the reservoir is supplied to the fluid pressure chamber so that the piston can be smoothly and rapidly retracted.
By the way, in the master cylinder disclosed in Japanese Unexamined Patent Publication No. 2003-194100, the thickness of the inner lip and the thickness of the outer lip are substantially equal. From this, it is considered that the inner lip and the outer lip both have the aforementioned sealing function and the pumping function. However, when the inner lip on which the piston slides has the two functions, the inner lip must be somewhat thin in order to ensure the pumping function of the inner lip. When the inner lip is formed to be thin, however, a problem that the inner lip enters in the relief port, i.e. is caught by the piston, is caused. For this, in the master cylinder disclosed in Japanese Unexamined Patent Publication No. 2003-194100, the inner lip is provided with a tapered surface formed on a side thereof on which the piston slides, thereby preventing the inner lip from being caught by the piston.
When the inner lip is provided with the tapered surface, however, there is a problem that rapid and secure supply of fluid to the fluid pressure chamber when the piston is retracted is impossible, that is, the property of supplying fluid should not be well. Further, the decrease in thickness of the inner lip makes the cap configuration easy to be deformed. Furthermore, since the inner lip has the tapered surface, there is also a problem that the sealing point of the inner lip is erratic.
To solve the problems in case that the both lips have the aforementioned two functions, it is considerable that the inner lip has only the sealing function while the outer lip has both the sealing function and the pumping function. When only the outer lip has the pumping function, however, fluid paths formed in the cylinder body must be complex in structure. Since fluid supply is conducted only by the outer lip, the fluid supply by the outer lip must cover the fluid supply to be originally conducted by the inner lip. Therefore, a larger amount of fluid to be supplied by the outer lip is required, thus making the fluid paths more complex. In addition, the cylinder must be composed of a number of divided parts in order to form such complex fluid paths so that there is a problem of increasing the number of parts.